Fluctuations of angle of attack and lift coefficient and the resultant fatigue loads for a large Horizontal Axis Wind turbine

Abdolrahim Rezaeiha,Ricardo Pereira,Marios Kotsonis

doi:10.1016/j.renene.2017.07.101

Abstract

Unsteady loads are a major limiting factor for further upscaling of HAWTs considering the high costs associated to strict structural requirements. Alleviation of these unsteady loads on HAWT blades, e.g. using active flow control (AFC), is of high importance. In order to devise effective AFC methods, the unsteady loading sources need to be identified and their relative contribution to the load fluctuations experienced by blades needs to be quantified. The current study investigates the effects of various atmospheric and operational parameters on the fluctuations of α and CL for a large HAWT. The investigated parameters include turbulence, wind shear, yawed inflow, tower shadow, gravity and rotational imbalances. The study uses the DTU's aeroelastic software HAWC2. The study identifies the individual and the aggregate effect of each source on the aforementioned fluctuations in order to distinguish the major contributing factors to unsteady loading. The quantification of contribution of each source on the total fatigue loads reveals >65% of flapwise fatigue loads is a result of turbulence while gravity results in >80% of edgewise fatigue loads. The extensive parametric study shows that the standard deviation of CL is 0.25. The results support to design active load control systems by highlighting the magnitude of CL and α variations experienced by HAWTs, and thus the dCL that needs to be delivered by an AFC system.

Highlights

Previous studies have shown that increasing the size of wind turbines is a driving factor towards more environmentally friendly and cost-efficient wind energy [1,2]
The results support to design active load control systems by highlighting the magnitude of CL and a variations experienced by Horizontal Axis Wind turbine (HAWT), and the dCL that needs to be delivered by an active flow control (AFC) system
0:3 as these inboard stations have a negligible contribution to aerodynamic loads felt at the blade root because of their somewhat cylindrical cross section and small effective velocity, and as such are not shown

Summary

Introduction

Previous studies have shown that increasing the size of wind turbines is a driving factor towards more environmentally friendly and cost-efficient wind energy [1,2]. Larger deflection for the slender blades may be a result of structural mode coupling due to unsteady, spatially inhomogeneous, wind fields and loads. This imposes strict requirements, leading to thicker blade laminates with higher structural strength and stiffness in order to withstand the inherently higher loads, due to the effects of gravity and non-uniform inflow conditions, and to limit the resultant larger deflections, leading to more expensive blades. In order to refrain these undesirable high costs, the sources of unsteady loads on the blades need to be identified and their resulting fatigue damage needs to be determined. Identification of the load fluctuations helps to effectively design the active load control systems in order to alleviate the identified fluctuations [3]: the dCL that needs to be delivered by an AFC system will be known

Methods

Results

Conclusion